The prior foreign art (German Offenlegungsschrift No. 2,056,733) describes a circuit arrangement for monitoring interruptions in each of two closed-circuit loops, which loops are laid in at least one cable, i.e. in one common cable, or in respective cables, wherein one end of each loop is connected, with the optional interposition of circuit elements, to one pole of a voltage source, the other end of each loop being connected, via a resistance network, to the other pole of the voltage source; potentials derived from points in the resistance networks being used to control an electronic switching element which changes its conductivity state in the event of an interruption in at least one loop.
In the aforementioned circuit arrangement, a direct voltage is used and an interruption in one loop leads to a rise in potential at a point in the associated resistance network, while an interruption in the other loop leads to a decrease in potential at a point in the other resistance network. In order for the increase and decrease of potential in each case to lead to an identical variation in the conductivity state of the same electronic switching element, in one case the directional sense of the change of potential is reversed by an interposed transistor acting as reversing amplifier, and the potentials derived from points in the two resistance networks, which serve as control signals, are decoupled by diodes and are fed to the common electronic switching element. The transistor which serves as a reversing amplifier and the decoupling diodes entail corresponding constructional expense.